1. Field of the Invention
The present invention relates generally to bipolar devices, and more particularly, to batteries having electrodes that are reticulated, or interdigitated, a controlled porosity, and to those that are perforated.
2. Description of the Related Art
Solid state energy devices, such as but not limited to lithium batteries or fuel cells, typically require high energy density as well as high power density. Power density can be related to the discharge rate, which can be a function of ion and electron transport rates. For example, an electrode in a lithium battery that is too thick can limit discharge rate because ion/electrode transport from the electrode to the interface with a separator, such as the electrolyte, can be rate limiting. On the other hand, if the electrode layers are very thin, then energy density suffers because the electrolyte, separator, and current collectors occupy a higher volume and contribute to a greater mass relative to the active material of the electrodes. In addition, the discharge rate can be limited by interface resistance allowing only a certain current rate per unit area of interface.
The lithium-ion and lithium-polymer rechargeable battery can be an attractive technology for rechargeable battery applications due to its high energy density, freedom in battery configuration, low potential for environmental and safety hazard, and low associated materials and processing costs.
Improvements in lithium rechargeable battery technology have occurred due to improvements in the storage materials used as the cathodes or anodes, or in the liquid or polymer electrolytes used with such batteries. Currently known cathode compounds such as LiCoO2 and LiMn2O4 when used with currently known anodes such as lithium metal or carbon have working voltages between about three and four eV. For many applications a high voltage and low weight are desirable for the cathode as this leads to high specific energy. For example, for electrical vehicle applications the energy-to-weight ratio of the battery determines the ultimate driving distance between recharging.
Research into lithium intercalation compounds that has been conducted thus far has focused primarily on the synthesis and subsequent testing of various oxide compounds. These efforts have led to the development of a variety of compounds, including LixCoO2, LixNiO2, LixMn2O4, and LixV3O13. In addition, LixTiS2 and other disulfides have been investigated for use in lithium intercalation.
Systems with multiple metals have been described in several patents and publications. Ohzuku, et al., “Synthesis and Characterization of LiAl1/4Ni3/4O2 for Lithium-Ion (Schuttle Cock) Batteries,” J. Electrochem. Soc., vol. 142, p. 4033 (1995), and Chiang et al., “High Capacity, Temperature-Stable Lithium Aluminum Manganese Oxide Cathodes for Rechargeable Batteries,” Electrochem. Sol. St. Lett., 2(3) pp. 107-110 (1999) describe the mixed-metal composition of the title and report electrochemical properties thereof. Cathodes in some rechargeable lithium batteries contain lithium ion host materials, electronically conductive particles to electronically connect the lithium ion hosts to a current collector (i.e., a battery terminal), a binder, and a lithium-conducting liquid electrolyte. The lithium ion host particles typically are particles of lithium intercalation compounds, and the electronically conductive particles are typically made of a substance such as carbon black or graphite. The resulting cathode includes a mixture of particles of average size typically on the order of no more than about 100 microns.
Anodes for rechargeable lithium-ion batteries typically contain a lithium ion host material such as graphite, a binder, and a lithium conducting liquid electrolyte. Alternatives to graphite or other carbons as the lithium ion host have been described by Idota et al., in Science 1997, 276, 1395, and by Limthongkul et al., in “Nanocomposite Li-Ion Battery Anodes Produced by the Partial Reduction of Mixed Oxides,” Chem. Mat. 2001.
In such cathodes or anodes, for reliable operation, good contact between particles should be maintained to ensure an electronically-conductive pathway between lithium host particles and the external circuit, and a lithium-ion-conductive pathway between lithium host particles and the electrolyte. To that, flooded electrolyte batteries have been used. Flooded electrolyte batteries are generally those wherein the electrodes are immersed in an electrolyte solution or matrix. This should improve performance by providing additional reaction sites.
Energy density can be intrinsically determined by the storage materials; the cell voltage can be determined by the difference in lithium chemical potential between cathode and anode, while the charge capacity can depend on the lithium concentration that can be reversibly intercalated by the cathode and anode. Power density, on the other hand, can be a transport-limited quantity, determined by the rate at which ions or electrons can be inserted into or removed from the electrodes.
Solid polymer electrolytes have been described. For example, Nagaoka, et al., in “A High Ionic Conductivity in Poly(dimethyl siloxane-co-ethylene oxide) Dissolving Lithium Perchlorate,” Journal of Polymer Science: Polymer Letters Edition, Vol. 22, 659-663 (1984), describe ionic conductivity in poly(dimethyl siloxane-co-ethylene oxide) doped with LiClO4. Bouridah, et al., in an article entitled, “a Poly(dimethylsiloxane)-Poly(ethylene-oxide) Based Polyurethane Networks Used as Electrolytes in Lithium Electrochemical Solid State Batteries,” Solid State Ionics, 15, 233-240 (1985) describe crosslinked polyether-grafted PDMS filled with 10 wt % LiClO4, and its ionic conductivity and thermal stability. Matsumoto, et al., in an article titled, “Ionic Conductivity of Dual-Phase Polymer Electrolytes Comprised of NBR-SBR Latex Films Swollen with Lithium Salt Solutions,” J. Electrochem. Soc., 141, 8 (August, 1994) describe a technique involving swelling poly(acrylonitrile-co-butadiene) rubber and poly(styrene-co-butadiene) rubber mixed latex films with lithium salt solutions resulting in dual-phase polymer electrolytes.
Electrodes for polymer batteries have also been described. For example, Minett, et al. in “polymeric insertion electrodes, Solid State Ionics, 28-30, 1192-1196 (1988)” describe a mixed ionic/electronic conducting polymer matrix formed by exposing a film of polyethylene oxide soaked in pyrrole to aqueous FeCl3 solution or by exposing a film of FeCl3-impregnated polyethylene oxide to pyrrole vapor. Films were assembled into all-solid-state electrochemical cells using lithium as the anode and PEO8LiClO4 as electrolyte. U.S. Pat. No. 4,758,483 (Armand) teaches of a solid polymeric electrolyte that can be used in a composite electrode. The electrolyte can include an ionic compound in solution in a copolymer of ethylene oxide and a second unit that can be an ethylene polyoxide structure including side-group radicals that introduce structural irregularities into the system reducing or eliminating crystallinity. A lithium salt, such as lithium perchlorate, can be dissolved in the polymer system.
While significant advances in battery formulations have been made, there is much room for improvement in increased power density and energy density in these types of devices.